Manufacture of polyolefins by monomer polymerization in a catalyzed gas phase process generally employs solid catalyst particles having defined morphology. Catalyst particle morphology in turn determines the morphology of the polymer particles as they grow around the catalyst particles.
Silica and other inert, inorganic materials have been used as supports for polyolefin catalysts, and have been used extensively for immobilizing metallocene-methylalumoxane catalysts. However, there are drawbacks to such simple, supported catalysts in that a) they are sometimes less active than the corresponding unsupported materials, and b) large particles (&gt;25 micrometers) of silica may remain in the finished polymer, where they may lower the polymer clarity and degrade the appearance rating of films made from the polymer.
One approach to avoiding the difficulties of supported metallocene catalysts is shown in U.S. Pat. No. 4,923,833, which discloses a spray dried metallocene catalyst composition containing no support material. In this patent, prepolymerization of olefin polymer around the spray dried catalyst composition particles is employed to achieve morphology control in the polymer product. This, however, brings disadvantages in time, effort and cost.
Metallocene/aluminoxane catalyst compositions containing no support material suffer from an additional drawback when using the solution-injection method of introducing such catalysts into the gas phase employed in olefin polymerization. Careful control of the catalyst flow rate, gas velocities and other reaction parameters is required or the morphology of the polyolefin particles produced becomes unstable. This may be manifested in the appearance of unacceptably large quantities of fines or agglomerates, sheeting, or fouling of the reactor. Such instability can lead to premature termination of reactor operation.
Accordingly, it would be desirable to have solid, metallocene catalyst compositions of good integrity and uniform morphology, and which do not result in large, residual silica particles in the product, yet whose activity is acceptable. Such catalyst compositions are the subject of this application.